1. Field of the Invention
This invention relates to a method and an apparatus for extruding a cross-linked material and more particularly to a method and an apparatus for extruding a cross-linked insulating material around an electric cable.
2. Description of the Prior Art
It is known in the art to make use as insulation for electrical cables of materials which are chemically cross-linked with peroxides, said cross-linkable materials being for instance polyethylen or its derivatives.
FIG. 1 schematically shows an extrusion machine for depositing a cross-linkable material on an electric cable.
In the known processes, the housing of the extrusion machine 1 comprises a feed screw rotatingly driven by a motor 2 for feeding a desired mixture 3 from a hopper 4 toward the downstream end 5 of the housing and toward the extruding head 6. A cable 7 is first coated with the cross-linkable material 8 before being forwarded toward the cross-linking oven 9.
It will be appreciated that, according to an important feature of the known apparatuses, in the feeding hopper 4 of the extrusion machine are simultaneously introduced all the materials provided for obtaining the cross-linked insulating material, i.e. the cross-linkable material, for instance polyethylen or its derivates, and the cross-linking agent, typically an organic peroxide. Accordingly, the temperature inside the housing of the extrusion machine has to be kept at relatively low values so as to prevent untimely cross-linking in the housing of the extrusion head, otherwise grains or heterogeneous particles would be present in the extruded materials. An additional requirement is that the time the materials while being forwarded remain in the housing and in the extrusion head be relatively short. It results therefore that the downstream filter or screen 10 of the extrusion machine cannot be as fine as required, and must be eventually suppressed. Thus, a very fine filter promotes the formation of dead zones in the housing of the extrusion machine within which a pre-cross-linking may occur.
Accordingly, a permanent drawback of the known processes resides in the fact that the cross-linkable material cannot be efficiently filtered after the outlet of the extrusion machine. Such a filtering is however an important requirement. As a matter of fact, the cross-linkable material is set in the hopper 4 in the form of a powder or granulates, said powder or granulates being generally obtained by cutting the cross-linkable material available in the form of rods or bars. Even if the cross-linkable material has been purified before being cut or minced, it results from the cutting or mincing steps that small metallic particles are mixed into the cross-linkable material. The ulterior presence of such small metallic particles, of a size for instance between 50 to 100 .mu.m may be irremediably prejudiciable to the good insulation of an electric cable. For instance, it can be said that the presence of a single metallic particle of a size of about 100 .mu.m in an amount of 1.5 tons of polyethylen necessary for insulating a cable of about 800 m in length is sufficient to be prejudiciable to a convenient operation of the cable. It results therefrom that, in the known processes, the thickness of the insulating liner has to be heavily dimensioned.
Another noticeable drawback of the known processes results in the fact that, as above mentioned, the temperature inside the housing of the extrusion machine has to be kept lower than the cross-linking temperature. Said temperature may be for instance about 130.degree. C. On the other hand, to carry-on efficiently cross-linking, the temperature must be raised at a value of about 160.degree. C. It is thus necessary, in the known apparatus, to achieve said raising in temperature downstream the extrusion dye, whereby it is necessary to increase the length of the cross-linking oven 9. It results therefrom an increased bulking of the fabrication plants.